Re: [CAD_CAM_EDM_DRO] Newb question....

John Trevick wrote:

>With all the questions about Stepper vs Servo what might be interesting to
>try is taking the Pico Systems USC with the Gecko Servo Interface and
>compare the performance of a Stepper motor vs a DC motor (I say DC motor
>instead of servo to try and avoid confusion). The point being to have a
>servo environment with as many elements the same like the software, encoder
>resolution, power supply size, etc. in order to measure the "efficiency"
>(for lack of a better term) of each motor. Perhaps using motors of the
>same frame size. I realize that the Gecko 2xx and 3xx series don't work at
>all the same way to move the motor but it would look the same to
>EMC. Also, I realize that Steppers and Servos don't usually work in the
>same voltage/amperage range but the bigger power supplies also cost more so
>it may be a fair handicap.
>

Actually, the power requirements for a servo system are often LESS than for
a stepper. DC servo motors are far more efficient than steppers. Idle
current
reduction helps the stepper come closer. DC servo motors operated in a
reasonable
range of their performance curve are VERY efficient. Steppers are
inherently
very inefficient, especially at low speeds, where their efficiency
approaches
zero.

> I suspect the result would be that the DC motor
>works better at medium to higher RPM but at least all the other elements
>which confuse the issue would be eliminated (like the general assumption
>that Stepper systems are open loop).
>
>

Ignoring any delays in microstepping and/or step multiplier logic, the
stepper
is "tighter" than most DC servos. What I mean by that is that forces
acting to
deflect the motor from the desired position can deflect a servo more easily,
unless the loop gain is extremely high. I think you'll find that Gecko
320 and 340
drives show this to a significant degree, at least with some motors. A low
power supply voltage seems to make this worse.

I've been working with my new PWM servo amps, and I believe they are a LOT
"tighter" than the Gecko 320. They go to the current limit at about 0.0005"
deflection with the setup I'm working on now on a Taig mill.

The big differences you won't see without forcing the system to the
breakdown
point. The steppers will breakdown, ie. lose steps and stall at a lower
torque
with increasing speed. The high-speed torque may be 1/10 of the holding
torque
in many cases. The servos will maintain full rated torque at any speed
up to
the point where the DC supply voltage minus the back EMF can no longer drive
enough current through the motor. A properly designed servo system has
enough
margin built into the design so that this "running out of voltage" never
happens
within the design velocity of the system.

Jon